The present invention relates to a method and a system for resistance seam welding of a foil and at least one foil support of a fuel cell system.
Diaphragm modules which contain metal separation diaphragms or foils are used in fuel cells which are operated using hydrogen which is extracted from hydrocarbons, methanol, for example. At low permeation rates metal separation foils function selectively and in a temperature-stable manner. In selective separation of hydrogen via a metal separation foil, the permeation rate depends on the foil material, the pressure, the temperature, and the foil thickness. The temperature range is generally between 250° C. and 450° C. No noteworthy hydrogen separation occurs below 250° C. Intermetallic structural transformations, which drastically deteriorate permeation, occur above 450° C. when palladium-copper alloys are used. The metal separation foils separate a high-pressure area from a low-pressure area in the diaphragm module. The higher the differential pressure between the two areas, the better the permeation rate. The differential pressure is limited by the strength of the foil. The efficiency of the fuel cell system decreases because a good deal of energy must be spent in producing high pressures. The thinner a foil, the higher the permeation rate. As a rule, metal separation foils have a thickness of less than 25 micrometer. The metal separation foils are framed for the use in diaphragm modules. In known systems, the metal separation foils are pressed with graphite seals or are sealingly joined with one another by spattering of blocking layers. The known systems have a carbon monoxide leak rate which is insufficient for commercial applications. If, in addition to hydrogen, carbon monoxide diffuses to the low-pressure side, then the required purity of the anode gas for the fuel cell system is not provided.
A hydrogen separator for a fuel cell reformer is described in Unexamined Patent Application DE 100 44 406 A1 in which a palladium foil having a thickness of 3–15 micrometer is applied to a mesh wire-shaped support structure via pressing or via rolling and pressing. The foil adjusts in part to the support structure, thereby increasing the effective surface, and the foil may expand and contract without ripping or forming creases. The wire mesh-shaped support structure has the disadvantage that, due to its waviness, the high-pressure and low-pressure sides of the fuel cell reformer cannot be operated reliably sealed from one another.
In principle, welding methods are to be considered for joining thin workpieces. Two workpieces are melted in each welding process, both workpieces being liquefied along a weld seam. Since, as a rule, the melting energy is supplied from one source such as, for example, a gas burner, an electric arc, or a laser light source, sufficient energy must be made available for both workpieces. If a foil and one or two foil supports of a fuel cell system are to be welded together, the problem arises that the joined pieces have different mass or thickness so that the foil would initially melt and run without being joined with the unmolten foil frame. Mechanically pressing the two pieces together is not an option in gas melt methods, electric arc methods, or in the use of laser light since the pre-stress produced must be in the welding area and thus molten, i.e., welded.
A thin separator foil is applied to a porous metal body in the hydrogen separator described in JP 08-215 551 A. A peripheral support frame is inserted in the edge area of the metal body to compensate for unevenness. The construction made up of separator foil, support frame, and metal body is joined with a bracket by hermetic welding, thereby creating a separation between a high-pressure area and a low-pressure area. Hermetic welding is carried out using a laser beam or an electron beam, a sealing weld seam being produced on the front side of the construction and on the bracket. The danger of defects exists due to the different thicknesses of the materials used, in particular in the area of the very thin separator foil. The reliability of the seal is affected. Furthermore, seam welding may be considered in joining thin workpieces. These methods are resistance welding methods. The workpieces to be joined are passed between two rollers. While the rollers rotate, they transfer force and current to the workpieces. A continuous linear seam is formed under constant current. The workpieces have electric resistance. Due to the specific resistance of the joining parts, heat is generated by the current flow, so that the necessary melting energy is released. In workpieces having different thicknesses, no melting away of the thinner workpiece occurs, since the workpieces are pressed together by the rollers. The rollers themselves are made of a metal having a high melting point and are actively cooled so that there is no fusion with the workpiece. Very thin workpieces cannot be welded to relatively thick workpieces using conventional seam welding methods, since the danger exists that the thin workpiece is damaged.